This invention concerns a photo-receiving, intensity-time conversion element or device which can be used in an exposure meter, an illuminometer or an image sensor.
FIG. 1 is a circuit diagram showing one example of such a conversion device in the prior art.
A photosensor 1 (a photodiode is used in this case) is a photocurrent conversion device producing a photocurrent i, wherein the value of i depends on the photo-receiving intensity L. A capacitor 2 is charged by said photocurrent i to integrate said photocurrent. Assuming the capacitance of the capacitor 2 is C, a voltage V.sub.1 as shown by the following equation is applied as an input to an inverter 3: EQU V.sub.1 =1/C.intg.idt (1)
In operation, the capacitor 2 is initially discharged to V.sub.1 =0, by means not shown, causing the inverter to produce an output "1". As the capacitor 2 starts to charge, the photo-receiving intensity L is determined by determining the period of time t.sub.0 until the output from the inverter 3 is turned to "0". Now, by setting the photocurrent as i=AL, where A is a constant, assuming that the current is approximately proportional to the intensity L, the input voltage V.sub.1 for the inverter 3 can be represented by the following equation: EQU V.sub.1 =ALt/C (2)
Then, by setting a threshold voltage for the inverter 3 as V.sub.th and the time required to arrive at the state V.sub.1 =V.sub.th as t.sub.0, the above equation (2) equals: EQU t.sub.0 =CV.sub.th /AL (3)
Since the photo-receiving intensity L usually varies in a wide range, if the intensity L varies by a factor of 10.sup.6 between its maximum and minimum levels, the conversion time t.sub.0 also varies by the factor of 10.sup.6 between its maximum and minimum values according to the above equation (3). This means that the maximum conversion time is as great as one second even if the minimum conversion time is set to one sec. Such a conversion time renders the device impossible for practical use. Thus, prior art devices suffer the drawback that the range for the photo-receiving intensity being capable of measurement is narrowed, which thereby narrows the dynamic range.